1. Field of the Invention
The invention relates to a device for generating a pulsed magnetic field, with a magnet to be operated in pulse operation which contains at least one superconductive refrigerant-free winding.
2. Description of the Related Art
Besides the metal superconductor materials known for a long time, for example NbTi or Nb3Sn, which have very low transition temperatures Tc and are therefore referred to as low-Tc superconductor materials or LTS materials, metal oxide superconductor materials with transition temperatures Tc above 77 K are also known. The latter materials are also referred to as high-Tc superconductor materials or HTS materials.
Attempts are also being made to construct superconducting magnet windings with conductors using such HTS materials. Owing to their still comparatively low current-carrying capacity in magnetic fields, particularly with a field strength in the tesla range, many of the conductors of such windings are still kept at a temperature level below 77 K, for example between 10 and 50 K, in spite of the intrinsically high transition temperatures Tc of the materials being used, so as to be able to carry significant currents at higher field strengths of for example a few tesla.
Special refrigerating units, for example in the form of so-called cryocoolers with a pressurized helium gas closed circuit, are preferably employed to cool the winding in the aforementioned temperature range. Such cryocoolers have the advantage that the refrigerating power is available without handling cryogenic liquids. The superconducting coil winding is only coupled by thermal conduction to the cold head of such a cryocooler, i.e. it is refrigerant-free per se.
In magnet systems of magnetic resonance machines, helium bath coolers are preferably employed. This, however, requires a significant liquid helium stock of the order of hundreds of liters. If the magnet becomes quenched, i.e. the magnet changes from the superconducting state to the normal conducting state because of a jump in temperature, then an undesired pressure build-up takes place in the cryostat as the helium evaporates.
Refrigerator cooling with the use of compounds having good thermal conductivity, for example in the form of correspondingly designed Cu tubes between the cold head of a refrigerating unit and the superconducting winding of the magnet, have furthermore been employed for LTS magnets. A disadvantage, however, is that depending on the distance between the cold head and the object to be cooled, the large cross sections necessary for thermal coupling lead to a significant increase in the cold mass. Particularly for applications in magnetic resonance machines, this constitutes a problem since spatially extended magnet systems necessarily require longer cooling times.
WO 03/098645 furthermore discloses a device of the type mentioned in the introduction in which a line system is provided, having at least one pipe in which a refrigerant flows while circulating according to the thermosiphon effect. The line system is coupled to the cold head of the refrigerating unit. The liquid refrigerant is supplied to the line system or pipe at the cold head. It flows downward in the line owing to the latter's gradient existing over the entire length of the pipe system. During this, it absorbs heat from the winding and evaporates. The evaporated refrigerant flows back up in the pipe, oppositely to the flow direction of the liquid refrigerant, and recondenses on a cold surface of the refrigerating unit or cold head. Circulation is thus set up inside the pipe.
The line system known from WO 03/098645 is used to cool the two open superconducting magnet windings of a magnetic resonance machine, which is designed as a so-called C magnet. The two separate coils lie vertically above each other, and are wound as an open cylindrical ring with a relatively large central opening. The two pipes provided in both described examples are respectively fed in the manner of a winding along the inner surface of the wound cylindrical coils, the pipe system being first supplied to one winding and then extending to the second winding. The cold head lies above the upper winding so that the evaporating refrigerant flows back up in the pipe system to the cold head, where it recondenses.
These coils are used to generate a constant basic magnetic field applied during the imaging in the scope of a magnetic resonance scan. They are thus operated statically. In certain applications, however, it is also necessary to operate superconducting magnetic field coils in a pulsed fashion, i.e. to generate and switch off the magnetic field in cyclic operation. One example of use for the field of magnetic resonance technology, when employing a low-field magnetic resonance machine which only generates fairly low basic magnetic fields in the range <0.3 T via its integrated superconducting magnet system, is the facility of also recording images based on high-field excitation. To this end, particularly for the described C magnet machines, it is necessary to be able to put one or two further magnets with superconducting windings, which can generate an additional magnetic field in the range >0.3 T, as required between the machine's own two magnets between which the patient is placed for the scan. In this case, it is then necessary to operate these coils in a pulsed fashion, i.e. the high magnetic field is generated and switched off again at very short time intervals. These are therefore AC or pulse coils. During the time when the high magnetic field is switched off, the image acquisition is carried out by using the machine's conventional imaging system.
A problem—independent of the aforementioned field of use—when using such AC or pulse coils, however, is that in any event the pulse operation causes large eddy currents in all the metal components of the cooling system, which may be made of copper. These cause ohmic losses which stress the cooling system. The eddy currents, which generate their own magnetic field, also cause deviations from the desired magnetic field profile of the coil, which impairs the imaging quality in magnetic resonance equipment.